Hormone - A hormone is a chemical secreted by an endocrine gland and carried in the bloodstream to target organs where it performs a specific function.

Functions of Hormones - The varying actions performed by hormones may be fast-acting (e.g. Adrenalin is a hormone that rapidly speeds up the heart and breathing rates when we get a fright), or may be slow-acting (e.g. Human Growth Hormone regulates the many body processes involved in growing from conception to death).

Endocrine Glands - These glands are ductless, and secrete hormones directly into the bloodstream.

Homeostasis - This is the tendency to maintain stability or uniformity in an organism's internal environment. A balance is maintained of temperature, and of chemicals such as water, salts and glucose.

A Stimulus-Response Model

Maintaining Human Body Temperature (37oC)

If body temperature rises above 37oC - The skin produces sweat which cools the body by using excess body heat to evaporate it. Also the body can consume cool drinks, find cool shade, radiate heat through the skin or by breathing out warm air, and by defaecation or urination.

If body temperature falls below 37oC - The muscles may shiver to produce heat through friction, skin muscles may contract to produce insulating air pockets next to the skin with 'goosebumps' and upright hairs, the body consumes warm drinks or finds warm shelter.

Maintaining Water Balance

Water is essential to the body for chemical reactions to take place within cells.

The amount of water consumed in food and drink should equal the amount of water lost through breathing out, sweating, urination and defaecation.